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The nominal diameter indicates the degree of enlargement of the shell-and-tube heat exchanger, while the product of the design pressure and the nominal diameter reflects the combination of enlargement and high operating parameters, and it directly determines the thickness of the rotating shell wall. The original intention (purpose) of specifying these parameters is to control the risks associated with the enlargement of shell-and-tube heat exchangers from both safety and performance perspectives. 1. Safety: The main feature that distinguishes shell-and-tube heat exchangers from other types of pressure vessels is the presence of a tube bundle inside the rotating pressure shell, which is responsible for heat transfer. Shell-and-tube heat exchangers are generally double-chamber pressure vessels; the tube bundle, as the component that bears the pressures, temperatures, and flow forces from both sides, has components such as the tube sheet, heat exchange tubes, and floating head cover. Failure of these components can lead to leakage of the fluids on both sides. This accordingly poses safety risks to construction and use. 1) Bundle alignment: The bundle components are generally installed as a whole within the rotating shell. Based on a heat exchange area of 80 m2/m3 per unit volume, the volumetric density of the metal content per unit volume is estimated to be 0.2 m3/m3, with a mass density of 1570 kg/m3. Based on this estimation, the weight of a tube bundle with a nominal diameter of 2600 mm and a heat exchange tube length of 6000 mm reaches 50 tons. To control the assembly risks associated with installing the tube bundle into the shell, based on engineering experience, the standards specify a minimum wall thickness for the shell that is different from that of ordinary containers, and also set stricter requirements for the roundness tolerance of the shell compared to ordinary containers ； For the tube bundle, specifications are provided for the outer diameter of the baffle plates and the allowable deviations, as well as requirements for the number and arrangement of tie rods. 2) Sealing structure: The structural design of large-scale heat exchangers needs to be chosen carefully; it is advisable to avoid using a tube sheet structure with flange connections as much as possible ； Fixed tube sheet heat exchangers should be given priority ； For fixed tube sheet heat exchangers and U-tube heat exchangers, the “b”-type tube sheet connection is recommended. For floating-head heat exchangers, the standard specifies that the maximum nominal diameter of such heat exchangers is 2600 mm ； For nominal diameters exceeding 2600 mm, the standards do not explicitly prohibit their use; however, large-diameter floating flanges and split hook-ring structures increase the risk of internal leakage, so greater caution is required when selecting them. 3) Fluid-induced vibration: The increasing size of heat exchangers raises the risk of fluid-induced vibration in the tube bundles, and it is difficult to predict such vibrations accurately ； Vibration involves fluid mechanics and dynamics; in engineering applications, complete design conditions are lacking, and the applicability of engineering experience is limited. The standards provide, in the form of appendices, the details for the design calculations related to fluid-induced vibrations; designers can carry out such calculations and make optimal adjustments as necessary, depending on the design conditions. 2. Performance: The performance of shell-and-tube heat exchangers mainly includes two aspects: flow and heat transfer. Unlike the flow in the tube side, the flow in the shell side involves both cross-flow and longitudinal flow, with the direction and velocity of flow constantly changing. Heat transfer and flow are thus quite complex, and accurately calculating the convective heat transfer coefficient and flow resistance of the fluid in the shell side represents a challenge in the process calculations for heat exchangers. The impact of enlarging heat exchangers on their performance includes the following main aspects: 1) Flow: Larger size corresponds to a higher processing capacity; as a result, the tube specifications in the shell side of the heat exchanger are larger, and the spacing between the baffle plates is also greater ； The hazards of fluid erosion in the inlet area of the tube bundle; vibrations induced by the fluid flowing through the tube bundle pose significant risks ； 2) Heat transfer: Due to the deviation in the flow in the shell side, the dead zones caused by stagnation are too large, the effective heat transfer area is reduced, and thus the heat transfer efficiency decreases. Large-scale heat exchangers are limited in number within a facility but are extremely important, and the industry attaches great importance to them. During the design phase, operating conditions should be clearly defined; design conditions and parameters should be determined reasonably; risks associated with scaling up should be evaluated; and advanced design methods should be employed to predict and control such risks.